Conventional progressive aspheric ophthalmic lenses carry "hidden" marks that allow the dispenser to identify the manufacturer of the lens and reconstruct the layout geometry of the lens. Two types of hidden marks are in conventional use.
One type of hidden mark ("R"- type marks) comprise slightly raised or slightly depressed, smooth-sided marks refract light rays very slightly. The R-type marks may be viewed under conditions wherein a high contrast border aligned with the sides of the mark acts as a reverse contrast background for the few rays of light that are bent by the mark to enter the eye. Areas adjacent to the R-type mark send either more or fewer light rays to the eye than does the area of the mark, thereby creating a visible difference in light intensity between the R-type mark and its surroundings. For example, hidden marks may be viewed by holding a marked lens at arms length and scanning the area of the hidden marks across the edge of a still more distant light source.
Another type of hidden marks ("S"- type mark) comprises a very slightly raised or very slightly depressed, rough sided mark that scatters light rays. The S-type marks may be viewed by flooding the marked lens with a directed high intensity light beam and viewing the lens in a direction out of the beam i.e. at a nonzero angle relative to the longitudinal axis of the beam so that only rays scattered out of the beam by the mark toward the viewer are seen. The S-type marks appear as bright spots against a dark background using the above technique. Alternatively, a light beam may be reflected from the marked surface to the observer's eye and the S-type marks will appear as dark figures against a light background.
The task of reading either type of mark is visually fatiguing and the viewer is placed in a position in which it is awkward to display the mark to another viewer or to add more marks, e.g. by marking the surface of the lens with ink, to the lens in a controlled manner.